Solid paper-mill waste residues amounting to thousands of tons per annum are usually disposed of in landfill sites creating a worldwide environmental problem. In addition, the use of recycled paper and its waste by-products increases the environmental implications of the paper manufacturing process. Removing the ink, clay, coatings and contaminants from waste paper in order to recover reusable cellulose fibers to produce recycled paper creates deinking sludge which in turn creates disposal problems. In addition, other non-cellulose by products obtained in the process of producing paper such as coatings, adhesives, dyes and fillers like calcium carbonate and clay wind up in the sludge creating an environmental problem and reducing the yield of cellulose production from paper mill waste.
In Europe only, eleven million tons of waste are produced yearly by the paper and wood industry (pulp and paper sludge), of which 70% originates from the production of deinked recycled paper. Wastes are very diverse in composition and consist of different types of sludge. In general, the paper sludge contains very high levels of dry solids because it is rich in fibers and therefore dewaters quite easily. All paper and wood waste (pulp, paper sludge, paper white and waste water) is a mixture of cellulose fibers (40-60% of dry solids), printing inks and mineral components (40-60% dry solids: kaolin, talc and calcium carbonate). The sludge from process water clarification is generated in the fiber recovery process from white waters and in the physical wastewater treatment process. It consists of mostly fines and fillers (both around 50%) depending on the recovered paper being processed [1,2]. A more extensive review on waste water characteristics is given by Pokhrel and Viraraghavan [3].
Thus, paper sludge disposal is a growing concern in the paper industry which is engaged in intensive research to develop alternative uses for waste paper sludge.
Due to legislation and increased taxes, landfills are eliminated as final destinations for wastes, and incineration with energy recovery is becoming the main waste recovery method. Other options such as pyrolysis, gasification, land spreading, composting and reuse as building material are being applied, although research and economic assessment is still needed for optimization of the processes [1]. Due to the large volumes of waste generated, the high moisture content of the waste and the changing waste composition as a result of process conditions, recovery methods are usually expensive and their environmental impact is still uncertain. For this reason, it is necessary to find alternatives and different applications of wastes, while taking into account the environmental and economic factors of these waste treatments. The second obstacle is the high amounts of acid required for the process.
Cellulose Whiskers also termed Nano Crystalline Cellulose (NCC) are fibers produced from cellulose under controlled conditions that lead to the formation of high-purity single crystals. They constitute a generic class of materials having mechanical strengths equivalent to the binding forces of adjacent atoms. The resultant highly ordered structure produces not only unusually high strengths but also significant changes in electrical, optical, magnetic, ferromagnetic, dielectric, conductive, and even superconductive properties.
The tensile strength properties of NCC are far above those of the current high volume content reinforcements and allow the processing of the highest attainable composite strengths. A review of the literature on NCC, their properties, and their possible use as a reinforcing phase in nanocomposite applications is given by Azizi et al [4].
One of the main obstacles in utilizing NCC in industrial applications is their relatively high price which is attributed mainly to the high energy that is needed to convert relatively large cellulose fibers and lignocellulose tissues to nano-scale fibers.